Eye Encyclopedia

Accumulates experience as you do. When fully charged it produces 15 Eyes of Q'bthulhu. Obtained at the Shadowy Market in Shadow Arenas.

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— Item Tooltip

The Eye Encyclopedia is a Tome bought in the Shadowy Market for 15 Hearts of Darkness.
When the Tome fully charges, it will grant the player 15 Eye of Q'bthulhu. Unlike Legendary Tomes (that can only be used once a week), these tomes can be used as often as you like without any cooldowns, however, they do not provide as many rewards as the Legendary Tomes per individual use.